Reciprocating Compressor System with Liquid Pumping Capability

ABSTRACT

A reciprocating-type piston compressor that can operate at very low speed (&lt;300 rpm) with the ability to pump liquids. It can be configured with one or more double-acting cylinders. A variable speed drive can be used to adjust the operating speed of the compressor to control system torque requirements throughout the compression cycle.

PRIORITY STATEMENT & CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Application Ser. No. 62/633,694, entitled “Wet Gas Compressor Drive System and Method” filed Feb. 22, 2018, in the names of Curtis Christopher Blundell and Douglas Harry Haines; which is incorporated by reference into this application in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present disclosure is related to the field of gas compressors for use in the oil and gas industry where liquids may be present in the stream, in particular, compressors used for casing gas, vapour recovery and multi-phase boosting, as some examples.

BACKGROUND OF THE INVENTION

The torque requirement for a conventional reciprocating compressor varies significantly throughout a single revolution due to the varying of the crank arm effective radius from zero at top and bottom dead centre to maximum at mid stroke, and due to the compression loads varying from negative at the beginning of the inlet stroke (expansion of gas in the clearance volume) to maximum at the end of the discharge stroke. The suction and discharge pressures also vary over time and circumstance, which also affects the torque requirement of the system. To minimize the size of the prime mover to the average power required by the compressor, the inertia of the system is used to both absorb and release power to the compression activity. When the compression power requirement is low, the excess power supplied by the prime mover speeds up the compressor slightly adding energy to the rotating system and when the requirement is higher than the prime mover can supply, the inertia of the system releases power to the compression activity slowing rotation slightly. This inertia is composed of moving masses of the system, the speed at which they are moving and, often, a large flywheel. As a faster rotating compressor has more inertia than a slower one, and given that the faster a compressor rotates, the displacement requirement is smaller and, thus, reducing the cost of the compressor, the industry trend in compressors is to rotate them as fast as possible subject to the mechanical and valve limitations of the compressor. Typically, such compressors are operated at 900 to 1800 rpm.

These high-speed compressors cannot tolerate small amounts of liquids without catastrophic failure because the pressure required to move liquids through the compressor valve is an order of magnitude higher than for a gas and, in addition, is higher than the mechanical components of the compressor can tolerate. When liquids are present in the gas feed to such compressors, the typical result is often catastrophic failure.

Natural gas produced from a gas well will, typically, contain some liquid, therefore, all compressors packaged for this service have mechanisms to separate liquids before the gas enters the compression element, and then pump the liquids back into the gas stream downstream of the compression element. The separation system consists of inertial, gravity and coalescing mechanisms coupled in series that are expensive, bulky and prone to control component failures and liquid pump failures. In addition, such systems are designed for the largest anticipated slug (100% liquid volume). Operationally, the slug capacity is usually underspecified and the separation system will fail when attempting to process a larger than anticipated slug thus resulting in the loss of production revenue reduction in addition to the cost of repair to the compressor caused by an oversize slug.

It is, therefore, desirable to provide a compressor that overcomes the shortcomings of the prior art and that can pump a 100% liquid phase indefinitely.

SUMMARY OF THE INVENTION

A reciprocating compressor system with liquid pumping capability is presented herein. In some embodiments, the compressor can comprise a slow-moving, low inertia system that can vary speed significantly over each revolution. In some embodiments, the compressor can comprise a variable frequency drive (“VFD”) to vary the speed of the compressor motor to maintain a constant power input. When the VFD senses the low power part of the cycle, the VFD can speed up the compressor to maintain constant power at a higher speed and lower torque (that is, the VFD can generate a higher frequency, lower amperage output to the motor). When the torque requirement increases, the VFD can sense the increased requirement and can maintain constant power by increasing torque and reducing speed (that is, the VFD can generate a lower frequency, higher amperage output to the motor). In the event liquid is introduced to the system, the VFD can slow the motor down to a speed consistent with moving liquids out of the compression chamber at a rate that is consistent with the power available from the drive and, thereby, avoid mechanical harm to the system.

In some embodiments, the compressor disclosed herein can allow a user to compress and transport process gas that is randomly fouled with liquid, while managing speed and torque demands. This can provide the advantage of allowing the user to extract gas from fields more efficiently with less power, and with less down time for maintenance and catastrophic failures.

Broadly stated, in some embodiments, a reciprocating compressor system with liquid pumping capability can be provided, the system comprising: a low inertia reciprocating piston compressor further comprising at least one intake valve and at least one discharge valve; an intake flow line operatively coupled to the at least one intake valve; an output flow line operatively coupled to the at least one discharge valve; a compressor drive unit operatively coupled to the compressor, the compressor drive unit configured to operate the compressor; and a suction pressure transducer operatively coupled to the intake flow line and further operatively coupled to the compressor drive unit, the suction pressure transducer configured to measure suction pressure within the intake flow line and to generate a suction pressure data signal, the compressor drive unit configured to control speed of operation of the compressor in response to the suction pressure data signal.

Broadly stated, in some embodiments, a method can be provided for compressing gas and pumping liquids, the method comprising the steps of: providing a reciprocating compressor system with liquid pumping capability, the system comprising: a low inertia reciprocating piston compressor further comprising at least one intake valve and at least one discharge valve, an intake flow line operatively coupled to the at least one intake valve, an output flow line operatively coupled to the at least one discharge valve, a compressor drive unit operatively coupled to the compressor, the compressor drive unit configured to operate the compressor, and a suction pressure transducer operatively coupled to the intake flow line and further operatively coupled to the compressor drive unit, the suction pressure transducer configured to measure suction pressure within the intake flow line and to generate a suction pressure data signal, the compressor drive unit configured to control speed of operation of the compressor in response to the suction pressure data signal; drawing in gas through the intake flow line and compressing the gas with the compressor; measuring the suction pressure in the intake flow line to produce a current suction pressure reading; comparing a historical suction pressure reading to the current suction pressure reading; and slowing the speed of operation of the compressor if the historical suction pressure reading is greater than the current suction pressure reading by a predetermined liquid warning threshold.

Broadly stated, in some embodiment, the method can further comprise slowing the compressor when a liquid is drawn into the at least one intake valve.

Broadly stated, in some embodiments, the compressor can comprise at least one double-acting cylinder.

Broadly stated, in some embodiments, wherein one or both of the at least one intake valve and the at least one discharge valve can comprise a check valve.

Broadly stated, in some embodiments, wherein the compressor can comprise at least one substantially horizontal cylinder wherein the at least one intake valve is disposed on a top surface thereof, and the at least one discharge valve is disposed on a bottom surface thereof.

Broadly stated, in some embodiments, the compressor drive unit can comprise a motor operatively coupled to a speed reducer, wherein the speed reducer is operatively coupled to the compressor.

Broadly stated, in some embodiments, wherein the motor can comprise an electric motor.

Broadly stated, in some embodiments, the system can comprise a variable frequency drive (“VFD”) unit configured for controlling speed of operation of the electric motor, wherein the VFD unit can be configured for operating the electric motor in a constant torque mode at motor speeds between 0 to 1800 revolutions per minute (“RPM”) and in a constant horsepower mode at motor speeds between 1800 to 3600 RPM.

Broadly stated, in some embodiments, the VFD unit can be configured to slow the speed of the electric motor when a liquid is drawn into the at least one intake valve.

Broadly stated, in some embodiments, the system can comprise a controller operatively coupled to the suction pressure transducer and to the VFD, the controller configured to generate and transmit a VFD control signal to the VFD, the VFD control signal configured for controlling the speed of the electric motor in response to the suction pressure data signal.

Broadly stated, in some embodiments, wherein the controller can be configured to compare a current suction pressure reading with a historical suction pressure reading and to generate a slug protection speed limit control signal if the historical suction pressure reading is greater than the current suction pressure reading by a predetermined liquid warning threshold.

Broadly stated, in some embodiments, wherein the controller can further comprise input controls configured for one or both of setting and controlling pressure limits and temperature limits of the system.

Broadly stated, in some embodiments, the controller can comprise a programmable logic controller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a block diagram schematic depicting one embodiment of a reciprocating compressor with liquid pumping capability.

FIG. 1b is a block diagram schematic depicting an alternate embodiment of the reciprocating compressor of FIG. 1 a.

FIG. 2a is a perspective view depicting one embodiment of the compressor of FIG. 1 a.

FIG. 2b perspective view depicting one embodiment of the compressor of FIG. 1 b.

FIG. 3 is an exploded, perspective view depicting the internal elements of the compressor of FIG. 2 a.

FIG. 4 is a side cross-section elevation view depicting the internal elements of FIG. 3.

FIG. 5 is a partial side elevation view depicting the compressor crankcase and associated drive of the compressor of FIG. 2 a.

FIG. 6a is an elevation view depicting a portion of one embodiment of the compressor of FIG. 2 a.

6 b is an elevation view depicting an alternate embodiment of the compressor of FIG. 6 a.

FIG. 7a is an X-Y data chart depicting plots of torque and motor speed of the compressor of FIG. 2a when no liquids are being pumped.

FIG. 7b is an X-Y data chart depicting plots of torque and motor speed of the compressor of FIG. 2a when liquids are being pumped.

FIG. 8 is a block diagram depicting functional features of the controller of FIG. 1 a.

FIG. 9 is a flowchart depicting a suction pressure control algorithm carried out by the controller of FIG. 8.

FIG. 10 is a flowchart depicting a liquid warning calculation algorithm carried out by the controller of FIG. 8.

FIG. 11 is a flowchart depicting a speed control algorithm carried out by the controller of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

In this description, references to “one embodiment”, “an embodiment”, or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etc. described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, the present technology can include a variety of combinations and/or integrations of the embodiments described herein.

Referring to FIG. 1 a, a functional schematic of one embodiment of the basic elements of wet gas compressor drive system 10 is illustrated (only left cylinder shown in schematic). In some embodiments, compressor drive system 10 can comprise compressor 40 and compressor drive unit 42. In some embodiments, compression cylinder 12 can enclose piston 14 connected to crosshead 22 by piston rod 20. In some embodiments, crosshead 22 can reciprocate inside tube 24 by means of connecting rod 28 attached to a pair of throw plates 32 rotated, via crankshaft 30, by electric motor 46. In some embodiments, torque produced by electric motor 46 can be transmitted through speed reducer 44. In some embodiments, electric motor 46 can be controlled and regulated by means of variable frequency drive (“VFD”) 48 wherein VFD 48 can be configured to receive and control an input supply of direct current (“DC”) electrical power or multi-phase alternating current (“AC”) electrical power to provide a supply of output electrical power to electrical motor 46, as well known to those skilled in the art. As drive 42 operates compressor 40, piston 14 can reciprocate within cylinder 12, thus drawing in gas or liquid from inlet 50 through intake valves 16, and then expelling same through discharge valves 18, and out through flow line 52. In some embodiments, one or both of intake valves 16 and discharge valves 18 can comprise check valves, as well known to those skilled in the art.

In some embodiments, system 10 can comprise suction pressure transducer 59 configured to measure the pressure of the gas/liquid mixture flowing through inlet 50. In some embodiments, suction pressure transducer 59 can be operatively coupled to controller 60 to provide pressure data of substances flowing in inlet 50 to controller 60. In some embodiments, VFD 48 can be operatively coupled to controller 60 wherein controller 60 can be configured to provide control signals to VFD 48 to control the electrical power being supplied by VFD 48 to electrical motor 46. Thus, controller 60 can receive pressure data from suction pressure transducer 59 to determine whether the substances flowing through inlet 50 comprise gas, liquid or a mixture thereof, and then provide control signals to the VFD 48 to control and regulate the electrical power supplied to electrical motor 46 accordingly. In some embodiments, controller 60 can comprise a programmable logic controller. In a representative embodiment, controller can comprise a model Micro820™ programmable logic controller as manufactured by Rockwell Automation of Milwaukee, Wis., USA or a model Simatic S7-1200 programmable logic controller as manufactured by Siemens AG of Nuremberg, Germany. In a representative embodiment, VFD 48 can comprise a model PowerFlex® 753 AC Drive as manufactured by Rockwell Automation of Milwaukee, Wis., USA or a model Sinamics G120 variable frequency drive as manufactured by Siemens AG of Nuremberg, Germany.

In some embodiments, there can be a potential risk of failure in system 10 if there is a failure of suction pressure transducer 59. Therefore, in some embodiments, mechanical torque limiting device 61 can be disposed in the mechanical coupling between motor 46 and speed reducer 44. In other embodiments, mechanical torque limiting device 62 can be disposed in the mechanical coupling between speed reducer 44 and compressor 40. This can either allow for controlled slip between the motor inertia and the compressor, or rapidly decouple the high inertia of the motor from the compressor. Either of torque limiting devices 61 and 62 can comprise a slipping type protection device, which could be in the form of a clutch, a fluid coupling or a magnetic coupling. In some embodiments, either of torque limiting devices 61 and 62 can be a disconnect type of coupling devices, which can comprise a ball detent or a shear pin coupling. In some embodiments, the disconnect device can require human intervention to reset, or can reset automatically, depending on operational requirements. In some embodiments, VFD 48 can comprise drive software configured to program controller 60 so that it can detect a slip condition of a disconnect device and, thus, bring motor 46 to a stop as quickly as possible. In some embodiments, controller 60 can be configured to either issue an alarm signal for reset or repair, or to automatically reset and resume operation of system 10 from low speed. In some embodiments, either or both of torque limiting devices 61 and 62 can provides a final mechanical protection in the event of a control system failure of system 10. In some embodiments, motor 46 can be coupled to torque limiting device 61 via belt drive 65 as shown in FIG. 1 b.

Referring to FIG. 2 a, shown is a top isometric external view of one embodiment of wet gas compressor drive system 10 comprising crankcase 36 with top pan 54 and bottom pan 56 attached thereto. As explained above, with the use of double-acting compressor 40, all left-hand sided elements are duplicated on their right, such as right sided cylinder 13, intake valves 17 and discharge valves 19. In some embodiments, pressure safety valve (“PSV”) 58 can join the inlet 50 to flow line 52 and can be used to release excess pressure from the system. In some embodiments, motor 46 can be coupled to speed reducer 44 via belt drive 65 as shown in FIG. 2 b.

Referring to FIG. 3, shown is a top isometric view of the internal elements of compressor 40, comprising crankcase divider 38, left-sided crosshead tube 24 and right-sided crosshead tube 25. Starting from common crankshaft 30, left-sided throw plates 32 can be positioned 90 degrees from right-sided throw plates 33. On the left side, throw plates 32 can be attached to their connecting rod 28 by throw pin 34, which can be attached to crosshead 22 by wrist pin 26, which can be further connected to piston 14 by piston rod 20. Equivalent right-hand sided components are shown and numbered accordingly, except for the right side crosshead pin 27, which can be disposed insideright crosshead 23. When assembled and enclosed, drive 42 (see FIGS. 1 & 2) can rotate crankshaft 30, causing throw plates 32 and 33 to reciprocate pistons 14 and 15 and, thereby, operate compressor 40.

Referring to FIG. 4, shown is a side cutaway view of compressor 40, with the figure elements as described above. FIG. 5 shows a partial side view of compressor crankcase 36 and associated drive 42. FIG. 6a shows a side view of drive 42 and an end view of crankcase 36 without its throwplate pans 54 and 56. FIG. 7 shows data plots of torque and motor speed performance results with the system under heavy load. In some embodiments, motor 46 can be coupled to speed reducer 44 via torque limiting device 61 as shown in FIG. 6 b.

Conventional compressors are high speed devices, with typical speeds ranging from 400 to 1800 rpm. In any piston compressor, the torque required to provide the force on the piston required to compress gas varies significantly over a revolution of the crank shaft. In a conventional high-speed compressor, the inertial nature of the system is such that it absorbs power by speeding up slightly during the part of the cycle when compression power requirements are low and releases this power to the compression activity when compression requirements are high thus slowing the system down slightly. Typically, these systems run with a speed and torque variation of less than 2%. Because of their high speed and high inertia, the introduction of relatively incompressible liquids can create a much higher pressure drop moving through the valves. This creates a very high-pressure spike that can be greater than the mechanical strength of the compressor components and catastrophic failure is often the result.

The disclosed wet gas compressor drive system 10, and its method of operation, is different from conventional gas compressors in that system 10 can comprise a low inertia reciprocating piston compressor by compressor 40 not having a flywheel. In some embodiments, system 10 can operate at a low speed as compared to conventional gas compressors so that the speed of the compressor can be varied significantly during a single revolution of the crank shaft as demonstrated by the torque vs. speed data shown in FIGS. 7a and 7 b. The masses of the components and the rotational speed of the compressor can be minimized as follows: Rotational power can be supplied by a conventional induction motor 46 through a low inertia planetary speed reducer 44, with motor 46 driven by VFD 48 that can be programmed for constant horsepower delivery from motor 46 through the 1800 to 3600 rpm range, and constant torque delivery from motor 46 during the 0 to 1800 rpm range. In a representative embodiment, a Teco Westinghouse PHD0154 15 horsepower, 480 volt 3-phase AC induction electric motor can be used although functionally equivalent motors that can be operated with a VFD, as well known to those skilled in the art, can be used. In some embodiments, the resulting output at crankshaft 30 can be less than 300 rpm depending on the amount of liquid in the gas flow. With normal gas flow, VFD 48 can sense lower torque demand of the system and adjust motor 46 speed to maintain either constant horsepower or constant torque. This torque-motor speed relationship is shown in FIG. 7 a. If a large quantity of liquid enters the gas compressor, the torque required to move the liquid through discharge valve 18 will rise, and the compressor speed can drop until an equilibrium is reached, with the speed of liquid moving through the valve being reduced to a point where compressor 40 has enough torque to move the liquid through discharge valve 18. This torque-motor speed relationship is shown in FIG. 7 b. One should note that the phase difference between torque and speed is due to minor inertial effects of the dynamic system.

In some embodiments, a permanent magnet AC motor or a DC motor could be used as motor 46 and directly coupled to a single stage planetary gear reducer or directly coupled to the input shaft of the compressor. In this embodiment, the inertia of the high speed induction motor described above would be significantly reduced, thereby making it easier for the drive to change speeds in response to changes in torque required.

In some embodiments, when there is no electrical power available to power an electrical motor embodiment of motor 46, an internal combustion engine driven pressure compensated or constant horsepower control on a hydraulic variable displacement pump and motor combination can be used as motor 46. This simpler torque limiting control system, such as a pressure compensated pump, can reduce speed quickly to maintain torque below operating limits to protect the machine components (bearings, crankshaft, connecting rods, etc.), and increase speed quickly to maximize production of gas.

In some embodiments, double acting compression cylinders, crosshead and piston rod can be used. In other embodiments, single acting pistons can be used, with or without a crosshead. In some embodiments, a crankshaft driving any number of reciprocating pistons can be used. In some embodiments, a low speed high torque driver can be used as speed reducer 44 in place of a high-speed motor and planetary gear reducer. In some embodiments, a low-speed, low-inertia driver coupled with a torque or power limiting device can be used.

In some embodiments, system 10 can comprise a power limiting drive system with a torque sensing variable speed drive to reduce motor speed quickly and to maintain power below operating limits when torque rises due to hazardous multiphase process conditions.

In some embodiments, system 10 can comprise a constant power drive so that as torque requirements change over time, the drive system can respond by increasing or decreasing speed to maintain constant power draw. This can be important, as power infrastructure is frequently limited at remote sites, thus, it is desirable to use the available power as efficiently as possible by minimizing the maximum power draw for a given operating condition.

In some embodiments, system 10 can comprise piston types with a geometry similar to a commonly available reciprocating compressors, with seals and materials compatible with both gas and liquid phases. This can increase utility and maintenance options in remote environments and, ultimately, increased operational longevity.

In some embodiments, system 10 can operate at low speeds (low gas velocities) and employs large ports (large valves) which make the novel compressor more tolerant to viscous liquids and solid particles that can be found in a typical operating environment.

In some embodiments, system 10 can comprise an intake/exhaust geometry that can eject liquids prior to the discharge of gas, unlike other prior art equipment on the market which retains liquids for sealing. In some embodiments, intake valves 16 and 17 can be disposed on a top surface of horizontal cylinders 12 and 13, and discharge valves 18 and 19 can be disposed on a bottom surface thereof, wherein liquids can pool on the bottom of cylinders 12 and 13 due to gravity and flow out through discharge valves 18 and 19 with the gas during compression cycle instead of coming out all at once at the end of the compression cycle, which is more difficult to manage. In some embodiments, system 10 can run completely dry if conditions call for it.

In some embodiments, system 10, as described herein, can comprise a potential shortcoming when the load of gas flowing therethrough is light and VFD 48 is consequently controlling electrical motor 46 to operate at maximum motor speed throughout much of the cycle. Introduction of large amounts of liquid under these conditions would require VFD 48 to slow motor 46 down significantly to be able to process the liquid as it has a much higher pressure drop through discharge valve 18 than gas. Under these conditions, compressor 40 is a low inertia device as it is turning slowly, whereas electric motor 46 is turning quickly, with its speed being reduced through speed reducer 44. Electric motor 46, thus, can comprise considerable stored energy, which is proportional to the square of the motor speed and it is possible that a breaking resistor operatively coupled to electrical motor and controller 60 can't react fast enough to the increase in torque which will occur due to liquid ingestion. In some embodiments, this could result in high forces and perhaps failure of the compressor components. To avoid this case, a method of determining that a large influx of liquid is in the piping moving towards the compressor (commonly called a slug) is provided. In some embodiments, the liquid moving towards compressor 40 can experience a much larger pressure drop traveling through the piping than an equivalent volume of gas and this can result in a decrease in the inlet pressure at the inlet to compressor 40 as the forcing pressure remains constant. This rapid reduction in pressure can be monitored via suction pressure transducer 59 and the software controlling VFD 48, either directly or through controller 60, can be configured to cause VFD 48 to start slowing motor 46 down prior to the liquid entering compression chamber 44 or 46, thus reducing the kinetic energy stored in motor 46 to a level low enough to prevent overstressing of the compressor components when discharging the liquid. Lower speed can also reduce the pressure required to pump liquids, further reducing the stress on components and risk of damage.

Referring to FIG. 8, one embodiment of the functional aspects or elements of controller 60 is shown. In some embodiments, suction pressure data measured by suction pressure transducer 59 at regular intervals can be relayed as suction pressure data signal 63 to controller 60 to measure the current suction pressure, as shown at current suction pressure function block 70. In operation, the suction pressure can range from 0 to 50 pounds per square inch (“PSI”) when only gas is flowing into system 10. When a slug of liquid is approaching inlet 50 of system 10, the suction pressure can be reduced by 5 to 10 PSI. Each successive suction pressure data signal 63 measured by transducer 59 can be relayed as suction pressure data signal 71 c and stored in a computer memory disposed in suction pressure history function block 72. At liquid warning calculation function block 76, the current suction pressure data reading, represented as suction pressure data signal 71 b, can be compared to a previous or historical suction pressure data entry, represented as previous suction pressure data signal 73. If the historical pressure reading minus the current pressure data reading is greater than a preset or predetermined liquid warning threshold, function block 76 can then generate slug protection speed limit control signal 77 that can be relayed to speed controller function block 78. In a representative embodiment, the preset or predetermined liquid warning threshold can be 10 PSI below the observed average pressing reading, although this can be selected to be higher or lower by those skilled in the art depending on the size of the compressor cylinders and on the relative composition of fluids and gas being processed from a particular well. At suction pressure control function block 74, the current suction pressure data entry at function block 70, represented as suction pressure data signal 71 a, can be used to send speed control signal 75 to speed controller function block 78. Function block 74 can comprise a scaler controller or a proportion-integral-differential (“PID”) controller to generate the speed control signal or any other speed controller method or process as well known to those skilled in the art, which can be configured to operate system 10 so as to maintain a minimum suction pressure as measured by suction pressure transducer 59. Speed controller function block 78 can receive speed control signal 75 from function block 74 in addition to receiving slug protection speed limit control signal 77 from function block 76. In some embodiments, controller 60 can comprise an additional control function block 80, which can comprise additional features, elements and/or input controls configured for setting and/or controlling additional pressure and temperature limits of system 10. These additional features, limits and/or controls can comprise controls or limits of subcomponents of system 10 or of other equipment used in combination with system 10 as would be understood by those skilled in the art. Thus, control function block 80 can generate additional control signal 81 that can then be relayed to speed controller function block 78. Thus, with input signals 75, 77 and 81 from any or all of function blocks 74, 76 and 80, speed controller function block 78 can be configured to generate VFD control signal 79 that can be relayed to VFD 48 so as to cause VFD 48 to set a minimum motor speed in motor 46.

Referring to FIG. 9, a flowchart is shown representing one embodiment of suction pressure control algorithm 900 that can be carried out by suction pressure control function block 74. In some embodiments, algorithm 900, starting at step 902, can compare suction pressure 908 to operator selected pressure set point 906 at decision step 904. If suction pressure 908 is less than set point 906, control function block 74 can then set the pump (“P”) speed to zero at step 910, and then exit algorithm 900 at step 916. For the purposes of this specification, “pump speed” shall mean the rotational speed of compressor 40 as shown in FIGS. 1a to 6 b. If suction pressure 908 is not less than set point 906, control function block 74 can then instruct PID control 912 to set the P speed to the output as generated by PID control 912 at step 914, and then exit at step 916.

Referring to FIG. 10, a flowchart is shown representing one embodiment of liquid warning calculation algorithm 1000 that can be carried out by liquid warning calculation function block 76. In some embodiments, algorithm 1000, starting at step 1002, can update section pressure 1004 at update suction pressure history step 1006 to keep an ongoing record or log of suction pressure at step 1010, which can be stored in a computer memory. At step 1008, a comparison of current suction pressure 1004 with a previous or historical suction pressure reading from suction pressure history 1010 can be undertaken. At decision step 1012, if the difference between a current suction pressure reading and a previous suction pressure does not exceed a predetermined liquid warning pressure differential threshold, then control function block 76 can exit algorithm 1000 at step 1016. If the difference between a current suction pressure reading and a previous suction pressure does exceed a predetermined liquid warning pressure differential threshold, then control function block 76 can then generate a liquid warning alarm condition at step 1014, and then exit at step 1016.

Referring to FIG. 11, a flowchart is shown representing one embodiment of speed control algorithm 1100 that can be carried out by speed control function block 78. In some embodiments, algorithm 1100 can, starting at step 1102, can determine if system 10 is in pause, alarm condition or shutdown mode at decision step 1104. If any of those conditions do exist, then control function block 78 can set the output speed of system 10 to zero at step 1106, and then exit algorithm 1100 at step 1116. If none of these conditions exist, then algorithm 1100 can determine whether a liquid warning alarm has been generated by algorithm 1000 at decision step 1108. If no liquid warning alarm has been generated, control function block 78 can maintain the pump speed at its current setting at step 1114, and can then exit algorithm at step 1116. If a liquid warning alarm has been generated, control function block 78 can determine whether the pump speed is greater than a liquid warning limit at decision step 1110. If the pump speed is not greater than the liquid warning limit, then control function block 78 can maintain the pump speed at its current setting at step 1114, and can then exit algorithm at step 1116. If the pump speed is greater than the liquid warning limit, then control function block 78 can reduce the pump speed to the speed limited by the liquid warning limit at step 1112, and can then exit algorithm 1100 at step 1116.

The various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the embodiments described herein.

Embodiments implemented in computer software may be implemented in software, firmware, middleware, microcode, hardware description languages, or any combination thereof. A code segment or machine-executable instructions may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc.

The actual software code or specialized control hardware used to implement these systems and methods is not limiting of the embodiments described herein. Thus, the operation and behavior of the systems and methods were described without reference to the specific software code being understood that software and control hardware can be designed to implement the systems and methods based on the description herein.

When implemented in software, the functions may be stored as one or more instructions or code on a non-transitory computer-readable or processor-readable storage medium. The steps of a method or algorithm disclosed herein may be embodied in a processor-executable software module, which may reside on a computer-readable or processor-readable storage medium. A non-transitory computer-readable or processor-readable media includes both computer storage media and tangible storage media that facilitate transfer of a computer program from one place to another. A non-transitory processor-readable storage media may be any available media that may be accessed by a computer. By way of example, and not limitation, such non-transitory processor-readable media may comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other tangible storage medium that may be used to store desired program code in the form of instructions or data structures and that may be accessed by a computer or processor. Disk and disc, as used herein, include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. Additionally, the operations of a method or algorithm may reside as one or any combination or set of codes and/or instructions on a non-transitory processor-readable medium and/or computer-readable medium, which may be incorporated into a computer program product.

Although a few embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications can be made to these embodiments without changing or departing from their scope, intent or functionality. The terms and expressions used in the preceding specification have been used herein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the invention is defined and limited only by the claims that follow. 

What is claimed is:
 1. A reciprocating compressor system with liquid pumping capability, the system comprising: a low inertia reciprocating piston compressor further comprising at least one intake valve and at least one discharge valve; an intake flow line operatively coupled to the at least one intake valve; an output flow line operatively coupled to the at least one discharge valve; a compressor drive unit operatively coupled to the compressor, the compressor drive unit configured to operate the compressor; and a suction pressure transducer operatively coupled to the intake flow line and further operatively coupled to the compressor drive unit, the suction pressure transducer configured to measure suction pressure within the intake flow line and to generate a suction pressure data signal, the compressor drive unit configured to control speed of operation of the compressor in response to the suction pressure data signal.
 2. The system as set forth in claim 1, wherein the compressor comprises at least one double-acting cylinder.
 3. The system as set forth in claim 1, wherein one or both of the at least one intake valve and the at least one discharge valve comprises a check valve.
 4. The system as set forth in claim 1, wherein the compressor comprises at least one substantially horizontal cylinder wherein the at least one intake valve is disposed on a top surface thereof, and the at least one discharge valve is disposed on a bottom surface thereof.
 5. The system as set forth in claim 1, wherein the compressor drive unit comprises a motor operatively coupled to a speed reducer, wherein the speed reducer is operatively coupled to the compressor.
 6. The system as set forth in claim 5, wherein the motor comprises an electric motor.
 7. The system as set forth in claim 6, further comprising a variable frequency drive (“VFD”) unit configured for controlling speed of operation of the electric motor, wherein the VFD unit is configured for operating the electric motor in a constant torque mode at motor speeds between 0 to 1800 revolutions per minute (“RPM”) and in a constant horsepower mode at motor speeds between 1800 to 3600 RPM.
 8. The system as set forth in claim 7, wherein the VFD unit is configured to slow the speed of the electric motor when a liquid is drawn into the at least one intake valve.
 9. The system as set forth in claim 7, further comprising a controller operatively coupled to the suction pressure transducer and to the VFD, the controller configured to generate and transmit a VFD control signal to the VFD, the VFD control signal configured for controlling the speed of the electric motor in response to the suction pressure data signal.
 10. The system as set forth in claim 9, wherein the controller is configured to compare a current suction pressure reading with a historical suction pressure reading and to generate a slug protection speed limit control signal if the historical suction pressure reading is greater than the current suction pressure reading by a predetermined liquid warning threshold.
 11. The system as set forth in claim 9, wherein the controller further comprises input controls configured for one or both of setting and controlling pressure limits and temperature limits of the system.
 12. The system as set forth in claim 9, wherein the controller comprises a programmable logic controller.
 13. A method for compressing gas and pumping liquids, the method comprising the steps of: providing a reciprocating compressor system with liquid pumping capability, the system comprising: a low inertia reciprocating piston compressor further comprising at least one intake valve and at least one discharge valve, an intake flow line operatively coupled to the at least one intake valve, an output flow line operatively coupled to the at least one discharge valve, a compressor drive unit operatively coupled to the compressor, the compressor drive unit configured to operate the compressor, and a suction pressure transducer operatively coupled to the intake flow line and further operatively coupled to the compressor drive unit, the suction pressure transducer configured to measure suction pressure within the intake flow line and to generate a suction pressure data signal, the compressor drive unit configured to control speed of operation of the compressor in response to the suction pressure data signal; drawing in gas through the intake flow line and compressing the gas with the compressor; measuring the suction pressure in the intake flow line to produce a current suction pressure reading; comparing a historical suction pressure reading to the current suction pressure reading; and slowing the speed of operation of the compressor if the historical suction pressure reading is greater than the current suction pressure reading by a predetermined liquid warning threshold.
 14. The method as set forth in claim 13, further comprising slowing the compressor when a liquid is drawn into the at least one intake valve.
 15. The method as set forth in claim 13, wherein the compressor comprises at least one double-acting cylinder.
 16. The method as set forth in claim 13, wherein one or both of the at least one intake valve and the at least one discharge valve comprises a check valve.
 17. The method as set forth in claim 13, wherein the compressor comprises at least one substantially horizontal cylinder wherein the least one intake valve is disposed on a top surface thereof, and the at least one discharge valve is disposed on a bottom surface thereof.
 18. The method as set forth in claim 13, wherein the compressor drive unit comprises a motor operatively coupled to a speed reducer, wherein the speed reducer is operatively coupled to the compressor.
 19. The method as set forth in claim 18, wherein the motor comprises an electric motor.
 20. The method as set forth in claim 19, wherein the system further comprises a variable frequency drive (“VFD”) unit configured for controlling speed of operation of the electric motor, wherein the VFD unit is configured for operating the electric motor in a constant torque mode at motor speeds between 0 to 1800 revolutions per minute (“RPM”) and in a constant horsepower mode at motor speeds between 1800 to 3600 RPM.
 21. The method as set forth in claim 20, wherein the system further comprises a controller operatively coupled to the suction pressure transducer and to the VFD, the controller configured to generate and transmit a VFD control signal to the VFD, the VFD control signal configured for controlling the speed of the electric motor in response to the suction pressure data signal.
 22. The method as set forth in claim 21, wherein the controller is configured to compare the current suction pressure reading with the historical suction pressure reading and to generate the slug protection speed limit control signal if the historical suction pressure reading is greater than the current suction pressure reading by the predetermined liquid warning threshold.
 23. The method as set forth in claim 21, wherein the controller comprises a programmable logic controller. 